Superfine bonded abrasive articles are typically used in microfinishing machines to provide a final polish to metal or ceramic articles. This microfinishing is generally accomplished by removing surface irregularities via a cutting action, which removes the roughness while maintaining form. Surfaces that have been microfinished with bonded abrasives are flatter, more parallel, have a higher load-carrying capacity, and may also be more true-running. Commonly used for this purpose are various vitrified bonded abrasive articles. However, there are several problems encountered in manufacturing fine grit bonded abrasives to form these articles.
One problem is that it is difficult to maintain homogeneous properties throughout the matrix of the bonded article. It has been observed that the abrasive "stone" hardness will often vary from point to point. A microstructural examination of these stones reveals that in some cases the variation in hardness is due to the presence of "bond spots". These bond spots are concentrated areas of grit and the bonding material, where porosity is reduced or absent. Bond spots behave like a larger grit in a fine grit stone because they break down much differently. Ultimately, these bond spots may produce scratches on the surface to be microfinished or polished.
Another problem encountered is that there may be variations in bond chemistry from point to point within an abrasive stone which reduce the homogeneity of the abrasive stone. The differences in bond chemistry alter the strength of the bond, thereby influencing the nonuniformity of the breakdown.
Another problem encountered is a variation in product from lot to lot. It has been observed that abrasive stones will often vary more in hardness and other measured properties from stone to stone than from point to point within a stone. This difference in hardness makes it difficult for the user to adjust the microfinishing machinery. The variation is often a result of the inability of the manufacturer to make greenware of consistent density and the manufacturer's use of impure raw materials that differ from lot to lot.
One way of preparing abrasive articles is disclosed in U.S. Pat. No. 2,942,991, which describes a slip casting process using colloidal silica for making refractories. U.S. Pat. No. 2,768,087 discloses silicon carbide abrasives that also use colloidal silica as a bonding agent. That patent describes mixing silicon carbide with a silica sol, fused alumina and ceramic raw materials capable of reacting with the silica of the silica sol to form a strong ceramic matrix. This matrix can then be sintered to form a bond. Among these raw materials are mixtures of clays of high pyrometric cone equivalent with an alkali metal silicate. Firing temperatures needed for this sintering are in the area of about 1450.degree. C.
Superfine grit abrasive articles can be prepared by a method known as "puddling." In this method a mixture of grit, fritted glass powder, clay and other raw materials, along with a green binder such as dextrin, is made into a slurry and put into a wooden form. This form is then placed into an oven and dried. The emerging block is shaved and fired. In the puddling process a density gradient can exist in the blocks due to the settling of coarser materials, and because raw materials that come from natural sources are used, the final product may also differ in composition. The result is varying bond strength which results in varying degrees of breakdown. In some cases the bond powders used in the puddling process are as large as or larger than the grit powders. Therefore, it is not unusual to observe concentrated areas of grit and bond, i.e., bond spots.
The uniformity of the final composition is also affected by its porosity. It is desired to have pores of uniform size and concentration throughout the article. It is difficult to control the porosity using methods such as puddling. Additives, such as coke, sawdust, walnut shell flour, and the like can be used to enhance this porosity, but may make the process more involved and ultimately affect the uniformity and performance of the abrasive article.
Another way of making abrasive articles is to cold-press a powder comprising a damp mixture of the grit, glass frit, clay and other raw materials along with a green binder such as dextrin. These mixes are made by blending the dry components with a small amount of water using a planetary mixer. The advantage of this procedure is that density can be controlled during the pressing operation, unlike in the puddling processes. However, it is difficult to achieve a completely homogeneous mixture and bond spots may result.
The above methods successfully produce abrasive articles, but do not solve problems resulting from non-homogeneity throughout the matrix or variation in product with each shipment. Thus, what is needed in the art is a method of producing abrasive articles, particularly superfine abrasive articles, and the abrasive articles themselves, that are of uniform, homogeneous composition, both within the abrasive stones and from lot to lot.